What version of Calibre-Web are you using? Mine has support to upload an image for a cover. I’m just using the latest docker image from ghcr.io/linuxserver/calibre-web.
My current workflow is pretty basic, I took my Calibre library I already had, dropped it into Calibre-Web and moved to a fully web based setup (so I don’t use Calibre at all anymore, just Calibre-Web). I’ve got the Kobo integration working too so I can mark books to sync to my device through the browser and then just sync my Kobo wirelessly.
I tend to use the Kobo store to purchase new books so I’m a bit lax about grabbing local copies of everything, but whenever I do get new books I can upload them into Calibre-Web, edit the metadata after I match the book, and then I’m good to go. But I also don’t add books that often at the moment, so haven’t seen much point in investing much time into further automation.
It’s an EPUB file that’s been optimized for a Kobo device.
And no, I don’t have Calibre running behind this anymore. I took the database Calibre (since I used it previously) had already created, loaded it with Calibre-Web, and it had enough capability that for me at least, I don’t need the desktop app anymore. I’m sure there’s any number of reasons why someone might, but for me, it works on its own.